You need to convert this to .Mp4 as Apple does not understand .MP3 files as Ringtones. You can download any of the free converter available online and then just put the ringtone to a folder and give the location to iTunes to pickup ringtones from the folder and then Sync it and you are good to go .

You don't need to get a converter.
All that ringtones are are AAC files, with the name changed to whatever.m4r.

So you can create a ringtone using ITunes; you just have to shorten the mp3 to thirty seconds or less along the way:

Start iTunes and find the song you want to convert. (It must be an MP3.)
Right-click the song and choose Get Info.
Click the Options tab.
Check the Start Time and Stop Time boxes, then enter times for each (no more than 30 seconds apart, the maximum length for a ringtone).
Click OK, then right-click the song again and choose Create AAC Version. You should immediately see a new 30-seconds-or-less version of the song.
Drag that version out of iTunes and into the folder of your choice.
Delete the 30-second version from iTunes and undo the Start Time/Stop Time changes to the original.
Open the folder containing the 30-second AAC file you dragged out of iTunes, then change the file extension from .m4a to .m4r. Double-click it and it immediately gets added to iTunes' ringtone library.
Finally, sync your iPhone. When it's done, you can head into the settings and select your new ringtone.

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